This invention relates to an electrical shutter control circuit of the type wherein the photocurrent of a light receiving element is integrated directly.
An electrical shutter control circuit of the type that light from an object is reflected by a film surface or a shutter curtain immediately before the film, and the reflected light is measured thereby to control the shutter has been proposed in the art. In the electrical shutter control circuit of this type, in general the photocurrent of the light receiving element caused by the light from the object which has passed through the photographing lens is integrated directly. The circuit is as indicated in FIG. 1. In the circuit shown in FIG. 1, a light receiving element 1 is connected between the input terminals of an operational amplifier 4, and an integrating capacitor 3 and a timing switch 2 which is opened in synchronization with the running of a shutter's top curtain are connected between the output terminal and the inverting input terminal of the operational amplifier 4. A constant current source 6 is connected to a variable resistor 7 which is set in accordance with an ASA sensitivity. This connection point and the output terminal of the operational amplifier 4 are connected to the input terminals of a comparator 5, the output terminal of which is connected to the base of a transistor 9 operating to drive a bottom curtain controlling magnet 8. Reference numeral 10 designates an electrical power source.
In operation, the light from the object passed through the photographing lens is reflected by the surface of the film or by the surface of the shutter curtain immediately before the film and reaches the light receiving element 1 to produce photocurrent. When the timing switch 2 is closed, the photocurrent flows in the timing switch 2, and therefore the output voltage of the operational amplifier 4 is zero volts which is lower than the terminal voltage of the variable resistor 7. As a result, the output of the comparator 5 is applied, as a base current, to the transistor 9 thereby to energize the bottom curtain controlling magnet 8. When, under this condition, the timing switch 2 is opened in synchronization with the running of the shutter top curtain, the aforementioned photocurrent is integrated in the integrating capacitor 3. When the voltage of the capacitor 3 reaches the terminal voltage of the variable resistor 7, the state of the comparator 5 is inverted, as a result of which the energization of the bottom curtain controlling magnet 8 is suspended. The period of time from the opening of the timing switch 2 to the suspension of energization of the bottom curtain controlling magnet 8 is the exposure time which will give a suitable exposure to the film.
If it is assumed that the photocurrent of the light receiving element 1 is represented by i.sub.p, the capacitance of the integrating capacitor 3 is represented by C.sub.0, and the terminal voltage of the variable resistor 7 is represented by V.sub.B, then the exposure time T can be expressed by the following equation (1): EQU T=C.sub.0 .multidot.V.sub.B /i.sub.p ( 1)
With respect to the object brightness B and the lens stop value A, the photocurrent i.sub.p can be expressed by the following equation (2): EQU i.sub.p =K.sub.0 .multidot.B/A.sup.2 ( 2)
where K.sub.0 is the proportional constant. PA1 where K.sub.1 is the proportional constant. PA1 where K.sub.2 =C.sub.0 .multidot.K.sub.1 /K.sub.0
With respect to the ASA sensitivity S, the terminal voltage V.sub.B which can be set in accordance with an ASA sensitivity can be expressed by the following equation (3): EQU V.sub.B =K.sub.1 /S (3)
Therefore, from equations (1), (2) and (3), the exposure time T can be expressed by the following equation (4): EQU T=K.sub.2 .multidot.A.sup.2 /B.multidot.S (4)
Accordingly, if the value K.sub.2 is suitably determined, the value T in equation (4) will be the exposure time during which the film is subject to suitable exposure. From equation (3), the terminal voltage V.sub.B of the variable resistor 7 is variable over a wide range of from 6.4 V to 3 mV or 2.1 V to about 1 mV (depending on the selection of the constant K.sub.1) for an ASA sensitivity range of from 3 to 6400. For an ASA setting of 6400, the electric shutter circuit is affected by the offset voltages or drift voltages of the operational amplifier 4 and the comparator 5, and therefore the accuracy of the circuit is very low. Furthermore, it is impossible to employ a low supply voltage 10 because of the factors involved when the ASA sensitivity is set to around ASA 3.